This invention relates to site-selective (toposelective) electrodeposition of a substance on a conductive component through the application of an electric field. More particularly, it is directed to such toposelective electrodeposition using bipolar electrochemistry. Applications include, by way of example but not limitation, site-directed partial or complete plating of a conductive substance, such as metal, conductive polymer or conductive salt, or a nonconductive substance, such as an electropolymerizable but non-conductive polymer, salt or adsorbate, on another material, such as a metallic particle; forming, in a manner analogous to "growing" a wire between two conductive contacts, where the wire formation may be three-dimensional; and forming electrical conductors on circuit boards or other electronic supports.
The construction of conductive paths represents a key element in electric circuitry and particularly in microcircuit fabrication. Photolithography and screen printing are currently the most widely used methods for creating circuitry on flat surfaces on most scales of commercial importance. For interconnect applications where coplanarity is not easily achieved, microsoldering or the application of a conductive adhesive is frequently employed. These approaches necessitate careful positioning of the soldering tool and physical contact with the circuitry.
There is a recent trend in materials science attempting to establish selective electrical contacts between conductive components without resorting to photolithographic techniques. To escape the limitations of planar circuit designs and to avoid physical contact, several novel approaches to circuit construction have been developed.
One such approach involves electrocrystallization (C. Gurtner, M. J. Sailor, Adv. Mater. 8:897 (1996)), or electropolymerization (C. L. Curtis, J. E. Ritchie, M. J. Sailor, Science, 262:2014 (1993)), from adjacent electrodes until an electrical connection is achieved by random physical contact of the growing conductive polymer or salt. In this way it has been possible to create conductive polymer-based diodes, transistors and signal amplifiers. (H. S. White, G. P. Kittlesen, M. S. Wrighton, J. Am. Chem. Soc. 106:5375 (1984)).
Other researchers have employed other techniques or strategies to construct conductive paths, such as templates (Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700); or on the surface of microelectrodes (G. P. Kittlesen, H. S. White; M. S. Wrighton J. Am. Chem. Soc. 107, 7373 (1985); (Martin, C. R. Science 1994, 266, 1961; Huber, C. A.; Huber, T. E.; Sadoqi, M.; Lubin, J. A.; Manalis, S.; Prater, C. B. Science 1994, 263, 800); scanning tunneling microscopy (W. Li, J. A. Virtanen, R. M. Penner Appl. Phys. Lett. 60, 1181 (1992)); thermally-driven strategies (von Gutfeld, R. J.; Vigliotti, D. R. Appl. Phys. Lett. 1990, 56, 2584); and contact electrodeposition strategies (Beck, A. F., Winter, J. U.S. Pat. No. 4,437,943). Rapid circuit prototyping has also been achieved with the use of "anti-fuses" which are activated by high applied potentials (Stopper, H.; Banker, J.; Miller, R. Proceedings of IMAPS International Conference on Multichip Modules,Denver, Col. Apr. 19-21, 1995, 191).
A major advantage of these approaches over photolithographic techniques is the possibility of forming contacts in three dimensions, thus greatly increasing the available information processing density currently available on two dimensional circuitry.
An electric field induces polarization in conductive particles. Beyond a critical polarization, the overpotential at the surface of the particle becomes sufficiently elevated to induce electrochemistry. Since each particle serves as both anode and cathode, the process is referred to as bipolar electrochemistry. This phenomenon has been investigated using fluidized or packed bed electrodes for applications in metal recovery, electrosynthesis and ultramicroelectrode studies. The technique is particularly well suited for electrochemistry in low conductivity media.
The process of this invention is based on bipolar electrochemistry and many aspects are based on spatially coupled bipolar electrochemistry (SCBE). SCBE is best explained by example. When a pair of electrodissolvable substances, such as copper particles, rings or the like are exposed to an electric field, they become polarized, even if they are not contacted by the electrodes forming the anode and cathode. At sufficiently elevated electric fields, material resulting from electrodissolution of the electrodissolvable substance, such as copper, for example, from one particle aligned with the electrodes becomes spatially coupled via electrodeposition on the other particle aligned with the first particle and the electrodes, resulting in the formation of a conductor or wire between the particles. Even without direct contact between at least one electrode and the particle and preferably, without any contact between either of the electrodes and the particles, and further, without initial contact between the particles, spatial coupling occurs between the particles. Thus, it has been discovered that the electrochemical phenomena between the particles are governed solely by the electric fields generated between the electrodes. SCBE allows the growth of conductive structures on isolated components, where contact with either or both electrodes is not required, with the location of the growth of the conductive substance being controlled by the electric field direction, rather than by contact of the electrodes with either or both particles involved in the reaction.
The present invention demonstrates that bipolar electrochemistry and SCBE are viable techniques for forming robust and adherent electrical connections in electric circuitry, including microcircuitry, involving forming (in essence, "growing") conductors using commercial circuit boards. An especially powerful feature of this invention is the formation of wires much smaller than the metallic components initially present without having to resort to photolithographic methods. The ability to predict and control wire growth between metallic or other electrodissolvable structures not directly connected to an external circuit represents a simple and cost effective approach to microcircuitry, including three dimensional microcircuit building. Selective wire formation within a matrix-bound ensemble of conductive components should be achieved by using three dimensional microelectrode arrays or even by the local electric field generated by linearly polarized light focused on a selected volume within the matrix. The use of a matrix may effectively reduce the fragility of the wires.
The disclosures of each of the references cited herein are hereby incorporated herein by reference.